The present invention relates to optical communication and, more particularly, to optical code division multiple access (OCDMA) communication in which signals encoded with a given code may be shifted to another code.
Various communications schemes have been used to increase data throughput and to decrease data error rates as well as to generally improve the performance of communications channels. As an example, frequency division multiple access (“FDMA”) employs multiple data streams that are assigned to specific channels disposed at different frequencies of the transmission band. Alternatively, time division multiple access (“TDMA”) uses multiple data streams that are assigned to different timeslots in a single frequency of the transmission band. However, FDMA and TDMA are quite rigid in the number of users and/or the data rates that can be supported for a given transmission band.
In many communication architectures, code division multiple access (CDMA) has supplanted FDMA and TDMA. CDMA is a form of spread spectrum communications that enables multiple data streams or channels to share a single transmission band at the same time. The CDMA format is akin to a cocktail party in which multiple pairs of people are conversing with one another at the same time in the same room. Ordinarily, it is very difficult for one party in a conversation to hear the other party if many conversations occur simultaneously. For example, if one pair of speakers is excessively loud, their conversation will drown out the other conversations. Moreover, when different pairs of people are speaking in the same language, the dialogue from one conversation may bleed into other conversations of the same language, causing miscommunication. In general, the cumulative background noise from all the other conversations makes it harder for one party to hear the other party speaking. It is therefore desirable to find a way for everyone to communicate at the same time so that the conversation between each pair, i.e., their “signal”, is clear while the “noise” from the conversations between the other pairs is minimized.
The CDMA multiplexing approach is well known and is explained in detail, e.g., in the text “CDMA: Principles of Spread Spectrum Communication,” by Andrew Viterbi, published in 1995 by Addison-Wesley. Basically, in CDMA, the bandwidth of the data to be transmitted (user data) is much less than the bandwidth of the transmission band. Unique “pseudonoise” keys are assigned to each channel in a CDMA transmission band. The pseudonoise keys are selected to mimic Gaussian noise (e.g., “white noise”) and are also chosen to be maximal length sequences in order to reduce interference from other users/channels. One pseudonoise key is used to modulate the user data for a given channel. This modulation is equivalent to assigning a different language to each pair of speakers at a party.
During modulation, the user data is “spread” across the bandwidth of the CDMA band. That is, all of the channels are transmitted at the same time in the same frequency band. This is equivalent to all of the pairs of partygoers speaking at the same time. The introduction of noise and interference from other users during transmission is inevitable (collectively referred to as “noise”). Due to the nature of the pseudonoise key, the noise is greatly reduced during demodulation relative to the user's signal because when a receiver demodulates a selected channel, the data in that channel is “despread” while the noise is not “despread”. Thus, the data is returned to approximately the size of its original bandwidth, while the noise remains spread over the much larger transmission band. The power control for each user can also help to reduce noise from other users. Power control is equivalent to lowering the volume of a loud pair of partygoers.
CDMA has been used commercially in wireless telephone (“cellular”) and in other communications systems. Such cellular systems typically operate at between 800 MHz and 2 GHz, though the individual frequency bands may only be a few MHz wide. An attractive feature of cellular CDMA is the absence of any hard limit to the number of users in a given bandwidth, unlike FDMA and TDMA. The increased number of users in the transmission band merely increases the noise to contend with. However, as a practical matter, there is some threshold at which the “signal-to-noise” ratio becomes unacceptable. This signal-to-noise threshold places real constraints in commercial systems on the number of paying customers and/or data rates that can be supported.
Recently, CDMA has been used in optical communications networks. Such optical CDMA (OCDMA) networks generally employ the same general principles as cellular CDMA. However, unlike cellular CDMA, optical CDMA signals are delivered over an optical network. As an example, a plurality of subscriber stations may be interconnected by a central hub with each subscriber station being connected to the hub by a respective bidirectional optical fiber link. Each subscriber station has a transmitter capable of transmitting optical signals, and each station also has a receiver capable of receiving transmitted signals from all of the various transmitters in the network. The optical hub receives optical signals over optical fiber links from each of the transmitters and transmits optical signals over optical fiber links to all of the receivers. An optical pulse is transmitted to a selected one of a plurality of potential receiving stations by coding the pulse in a manner such that it is detectable by the selected receiving station but not by the other receiving stations. Such coding may be accomplished by dividing each pulse into a plurality of intervals known as “chips”. Each chip may have the logic value “1”, as indicated by relatively large radiation intensity, or may have the logic value “0”, as indicated by a relatively small radiation intensity. The chips comprising each pulse are coded with a particular pattern of logic “1”'s and logic “0”'s that is characteristic to the receiving station or stations that are intended to detect the transmission. Each receiving station is provided with optical receiving equipment capable of regenerating an optical pulse when it receives a pattern of chips coded in accordance with its own unique sequence but cannot regenerate the pulse if the pulse is coded with a different sequence or code.
Alternatively, the optical network utilizes CDMA that is based on optical frequency domain coding and decoding of ultra-short optical pulses. Each of the transmitters includes an optical source for generating the ultra-short optical pulses. The pulses comprise Fourier components whose phases are coherently related to one another. A “signature” is impressed upon the optical pulses by independently phase shifting the individual Fourier components comprising a given pulse in accordance with a particular code whereby the Fourier components comprising the pulse are each phase shifted a different amount in accordance with the particular code. The encoded pulse is then broadcast to all of or a plurality of the receiving systems in the network. Each receiving system is identified by a unique signature template and detects only the pulses provided with a signature that matches the particular receiving system's template.
The known optical CDMA networks that use chip patterns or phase coding, however, require that the encoding applied at the transmitter be matched to the decoding applied at the desired receiver in order for the receiver to extract the coded signals sent by the transmitter. As a result, random interconnections between a given transmitter and a given receiver are not possible.
FIG. 1 depicts, in block diagram form, a known multiple user system 100. Such a system is described in U.S. Pat. No. 4,779,266, issued Oct. 18, 1988 to Fan R. K. Chung, et al. and titled “Encoding And Decoding For Code Division Multiple Access Communication Systems”, the disclosure of which is incorporated herein by reference.
The multiple user system 100 includes M sources 101, . . . ,103 that are arranged to communicate with N receivers 111,112, . . . ,113 over an interposed optical channel 141. The sources 101,102, . . . ,103 are coupled to the channel 141 via electro-optical encoders 121,122, . . . ,123. At the receiving end, electro-optical decoders 131,132, . . . ,133 couple the channel signals to the receivers 111,112, . . . ,113, respectively. Each encoder 121,122, . . . ,123, besides performing an encoding function, also converts electrical input signals to optical output signals. Similarly, each decoder 131,132, . . . ,133, in addition to its decoding function, also converts optical input signals to electrical output signals. The optical portion of system 100 is shown generally as between the dashed lines that intersect, respectively, the encoder blocks and the decoder blocks.
The optical channel 141 propagates only two-level or two-state digital signals, such as a logic “0” (a “space”) and a logic “1” (a “mark”). To match this channel characteristic, signals emanating from encoders 121, . . . ,123 via leads 151, . . . ,153, designated by signature signals Si, where i=1, . . . ,M, respectively, provide a stream of two-level or mark and space signals. Each Si stream corresponds to a similar stream produced by a particular one of the sources 101, . . . ,103. Because the channel 141 only supports two-level signals, if one or more of the encoders 121, . . . ,123 propagates logic “1” signals over channel 141 during the same time duration, the channel level remains at logic “1”. The channel level is at logic “0” if all the Si outputs are “0” during the same duration. In a logical sense, channel 141 behaves as an “inclusive OR” channel.
The composite signal on channel 141 resulting from all of the Si's is the summation of all the Si's and is represented by S0, where the summation is treated in the “inclusive OR” sense. Each lead 161, . . . ,163 emanating from the channel 141 serves as an input to the decoders 131, . . . ,133 and provides the composite signal S0. Thus, all the signatures Si share substantially the same frequency band on channel 141.
Generally, each signature signal Si is unconstrained in time in that each source 101, . . . ,103 may initiate a transmission or an interchange of information at any time independent of the other sources. Thus, synchronization between or among the autonomous sources 101, . . . ,103 is not required. However, each of the encoders 121, . . . ,123 is in synchronism with its corresponding source 101, . . . ,103.
Typically, one or more of the decoders 131, . . . ,133 are in synchronism with a predetermined encoder 121, . . . ,123. Each encoder may “train” its associated decoder using any known training techniques to provide the requisite synchronization. Additionally, synchronization between or among the autonomous decoders 131, . . . ,133 is not required, but each receiver 111, . . . ,113 is synchronized with its associated decoder.
The primary function of the encoders 121, . . . ,123 is to convert each logic “1” received from each corresponding source 101, . . . ,103 to a predetermined rate-increased stream of logic “1”s and logic “0”s, as depicted generically in FIG. 2. Line (i) of FIG. 2 depicts three contiguous data bits, namely, a “mark-space-mark” sequence, e.g., appearing in the output stream of source 101 or in the input stream to encoder 121. The time interval of either a mark or space is designated as a bit duration. Line (ii) of FIG. 2 represents an output pulse stream, e.g., Si from encoder 121, corresponding to the line (i) input stream. A rate-increased stream of logic “1” and logic “0” pulses, which is replicated for all other marks produced by the source 101, is generated by encoder 121. Because the channel 141 is an optical medium, the logic “1” levels in the output stream Si correspond to light pulses.
In the rate-increased or optical portion of the system 100, a frame corresponds to a bit duration, and the time interval of a logic “1” (a light pulse) or a logic “0” (no light pulse) is designated as the chip duration. Thus, each frame is composed of a fixed number of “chips”, e.g., three logic “1” chips occur during each mark frame in FIG. 2. The envelope of the mark frames is shown by the dashed rectangles on line (ii) of FIG. 2.
To communicate effectively within the system 100, each signature Si, as produced by its assigned encoder in response to an input mark, may not be selected arbitrarily but must be carefully chosen to achieve efficient, error-free communication. Specifically, each Si must be selected in view of all the other Si's based on considerations such as the number of sources M and the bandwidth of the channel 141. The considerations, in turn, depend on the communication or system requirements or the transmission characteristics.
Each of the decoders 131, . . . ,133 discriminates the pre-assigned signature associated with that decoder from within the composite signal S0. As an example, each of the decoders 131, . . . ,133 may be implemented using optical tapped delay lines arranged along channel the 141. The optical separation between the taps of each decoder corresponds to the distribution of logic “1” chips in the signature that is pre-assigned to that decoder. Thus, whenever a mark is transmitted, each tap in a given decoder extracts a high-peak signal when the logic “1” chips in the pre-assigned signature propagating as part of S0 are aligned with the taps. In this way, a “peak correlation” reveals the arrival of the pre-assigned signature and, in turn, the propagation of a mark by the source having the same pre-assigned signature.
The known optical system of FIGS. 1-2 has the disadvantage that in order for a given one of the M sources 101, . . . ,103 to communicate with one or more of the N receivers 111, . . . ,113, the pre-assigned signature associated with its corresponding decoder 131, . . . ,133 must be generated by the encoder 121, . . . ,123 corresponding to the given transmitter. As a result, random interconnections between a given source and a given receiver are not possible.
It therefore desirable to provide an optical CDMA system in which signals encoded by a given transmitter may be readily converted to another code so that the coded signals may be decoded by a desired receiver whose decoding is not matched to the transmitter.